DTV broadcasting in the United States of America has been done in accordance with broadcasting standards formulated by an industry consortium called the Advanced Television Systems Committee (ATSC). ATSC published a Digital Television Standard in 1995 that employed 8-level vestigial-sideband amplitude modulation of a single radio-frequency (RF) carrier wave. This DTV transmission system is referred to as 8VSB. In the beginning years of the twenty-first century efforts were made to provide for more robust transmission of data over broadcast DTV channels without unduly disrupting the operation of so-called “legacy” DTV receivers already in the field. These efforts culminated in an ATSC standard directed to broadcasting digital television and digital data to mobile receivers being adopted on 15 Oct. 2009. This subsequent standard also used 8-level vestigial-sideband amplitude modulation of a single RF carrier wave, so the more robust transmission of data could be time-division multiplexed with the transmission of DTV signal to so-called “legacy” DTV receivers already in the field.
DTV broadcasting in Europe has employed coded orthogonal frequency-division multiplexing (COFDM) that employs a multiplicity of RF carrier waves closely spaced across each 6-, 7- or 8-MHz-wide television channel, rather than a single RF carrier wave per television channel. Adjacent carrier waves are orthogonal to each other. Successive multi-bit symbols are selected from a serial data stream and used to modulate respective ones of the multiplicity of RF carrier waves in turn, in accordance with a conventional modulation scheme—such as quaternary phase shift keying (QPSK) or quadrature amplitude modulation (QAM). QPSK is preferably DQPSK, using differential modulation that is inherently insensitive to slowly changing amplitude and phase distortion. DPSK simplifies carrier recovery in the receiver. Customarily, the QAM is either 16QAM or 64QAM using square 2-dimensional modulation constellations. In actual practice, the RF carrier waves are not modulated individually. Rather, a single carrier wave is modulated at high symbol rate using QPSK or QAM. The resulting modulated carrier wave is then transformed in an inverse fast discrete Fourier transform (I-DFT) procedure to generate the multiplicity of RF carrier waves each modulated at low symbol rate.
In Europe, broadcasting to hand-held receivers was done using a system referred to as DVB-H. DVB-H (Digital Video Broadcasting-Handheld) is a digital broadcast standard for the transmission of broadcast content to handheld receivers, published in 2004 by the European Telecommunications Standards Institute (ETSI) and identified as EN 302304. DVB-H, as a transmission standard, specifies the physical layer as well as the elements of the lowest protocol layers. It uses a power-saving technique based on the time-multiplexed transmission of different services. The technique, called “time slicing”, allows substantial saving of battery power. Time slicing allows soft hand-over as the receiver moves from network cell to network cell. The relatively long power-save periods may be used to search for channels in neighboring radio cells offering the selected service. Accordingly, at the border between two cells, a channel hand-over can be performed that is imperceptible by the user. Both the monitoring of the services in adjacent cells and the reception of the selected service data can utilize the same front end.
In contrast to other DVB transmission systems, which are based on the DVB Transport Stream adopted from the MPEG-2 standard, the DVB-H system is based on Internet Protocol (IP). The DVB-H baseband interface is an IP interface allowing the DVB-H system to be combined with other IP-based networks. Even so, the MPEG-2 transport stream is still used by the base layer. The IP data are embedded into the transport stream using Multi-Protocol Encapsulation (MPE), an adaptation protocol defined in the DVB Data Broadcast Specification. At the MPE level, DVB-H employs an additional stage of forward error correction called MPE-FEC, which is essentially (255, 191) transverse Reed-Solomon (TRS) coding. This TRS coding reduces the S/N requirements for reception by a handheld device by a 7 dB margin compared to DVB-T. The block interleaver used for the TRS coding creates a specific frame structure, called the “FEC frame”, for incorporating the incoming data of the DVB-H codec.
The physical radio transmission of DVB-H is performed according to the DVB-T standard and employs coded orthogonal frequency division multiplexed (COFDM) multi-carrier modulation. DVB-H uses only a fraction (e.g., one quarter) of the digital payload capacity of the RF channel. DVB-H uses two-dimensional Reed-Solomon coding of randomized data followed by convolutional coding in generating signal for mapping to QAM symbol constellations that modulate COFDM carriers. Reed-Solomon (RS) coding is a special case of Bose-Chaudhuri-Hocquenghem (BCH) coding that uses multiple-bit symbols. DVB-T employs COFDM in which an 8 MHz-wide radio-frequency (RF) channel comprises approximately 2000 or approximately 8000 evenly-spaced carriers for transmitting to stationary DTV receivers. DVB-T2, a replacement for DVB-T proposed in 2011, further permits approximately 4000 evenly-spaced carrier waves better to accommodate transmitting to mobile receivers. DVB-T2 uses BCH coding of randomized data followed by low-density parity-check (LDPC) coding to generate signal for mapping to QAM symbol constellations that modulate COFDM carriers.
COFDM has been considered more than once for DTV broadcasting in the United States of America. It was considered as a replacement for 8VSB at the time that the ATSC Digital Television Standard was updated to permit more robust transmissions for reception by mobile receivers. At that time any technical advantages of COFDM were over-ridden by the need not to obsolete DTV receivers already in the field, lest advertising-supported over-the-air DTV fail as a commercially viable business. The invention, which concerns remedying low densities of ONEs in packets of digital data that are to be coded for broadcasting, can be employed in DTV broadcasting whether COFDM of multiple RF carriers or 8VSB amplitude-modulation of a single RF carrier is used. The detailed description of the invention infra refers particularly to a DTV broadcast system employing COFDM, since it is more likely that COFDM will be reconsidered as a replacement for 8VSB DTV broadcasting in the USA than changes will be made in 8VSB DTV broadcasting which would obsolete DTV receivers already in the field.
Packets of digital data for DTV broadcasting quite often include long sequences of ZEROs. Low densities of ONEs in packets of digital data for DTV broadcasting tend to cause problems with generating codewords with large Hamming distances—i.e., codewords that differ in so many bit-places from each other as to be distinguishable readily one from another. Codewords with large Hamming distances are better received than codewords with smaller Hamming distances during reception over Rayleigh channels. A customary approach taken to overcome low densities of ONEs in digital data is to exclusive-OR a pseudo-random binary sequence of some length with the digital data in serial-bit form. Such procedure converts long sequences of ZEROs in packets of digital data to mixed ONEs and ZEROs. Packets of digital data for DTV broadcasting are unlikely to be particularly correlated with the pseudo-random binary sequence that they are exclusive-ORed with, so the resulting randomized digital data is unlikely to include long sequences of ZEROs.
Any long sequences of ZEROs that do remain, however, can be eliminated by ONEs' complementing just the packets of digital data containing them. If the bits of only selected packets of digital data are ONEs' complemented, the transmitter has to signal the receiver which packets of digital data have their bits ONEs' complemented. It would be desirable that such signaling use as little as possible of the digital bandwidth available for transmissions, but at the same time provide signaling that is highly reliable.
The reader should note that COFDM transmitters broadcasting according to DVB-T and DVB-H standards employ (204, 188) Reed-Solomon coding of digital data packets as an outer coding procedure performed before byte-interleaving and subsequent inner coding. This is analogous to the (207, 187) Reed-Solomon coding of digital data packets as an outer coding procedure that 8VSB DTV transmitters employ before byte-interleaving and subsequent inner coding. The RS coding of digital data packets as an outer coding procedure prior to byte-interleaving and subsequent inner coding is an essential ingredient in the remedy for low densities of ONEs in packets of digital data that is to be described.
Reed-Solomon codewords that are not shortened have an interesting property in that, when all their bits are ONEs' complemented, other Reed-Solomon codewords result. So, if the channel symbols have been inverted somewhere along the line, the RS decoders will still operate. The result of decoding will be the complement of the original data. This code property, which has been referred to as “transparency”, is lost when the Reed-Solomon (RS) code is shortened. The “missing” or “virtual” bits in a RS shortened code need all to be filled either by ONEs or by ZEROs, depending on whether the data is ONEs' complemented or not. To put it more precisely, if the symbols in a shortened RS code are inverted, then the “virtual” bits that were ZEROs need to be inverted to “virtual” bits that are ONEs when decoding the shortened RS codes. Conventional wisdom is that therefore it is mandatory that the sense of the data (i.e., TRUE or complemented) be resolved before decoding shortened RS codes.
Unless shortened, BCH codewords in general are “transparent”, so that when all their bits are ONEs' complemented, other BCH codewords result. “Transparency” is lost when the BCH code is shortened. In order to regenerate a full and complete BCH codeword for decoding purposes, the “missing” or “virtual” bits in a shortened BCH code need all to be filled either by ONEs or by ZEROs, depending on whether the data is ONEs' complemented or not. To put it more precisely, if the symbols in a shortened BCH code are inverted, then the “virtual” bits that were ZEROs need to be inverted to “virtual” bits that are ONEs when decoding the shortened RS codes. Still more generally considered, the “virtual” bits in a shortened BCH code can be any binary code or the ONEs' complement of that binary code, depending on whether the data is ONEs' complemented or not. In this specification and its claims, filling all the “missing” or “virtual” bits in a shortened BCH code by ZEROs is referred to as “ZEROs-fill technique”, and filling all the “missing” or “virtual” bits in a shortened BCH code by ONEs is referred to as “ONEs-fill technique”.